[SECURITY PATCH 02/30] commands/boot: Add API to pass context to loader

[Daniel Kiper]

From: Chris Coulson <chris.coulson@canonical.com>

Loaders rely on global variables for saving context which is consumed
in the boot hook and freed in the unload hook. In the case where a loader
command is executed twice, calling grub_loader_set() a second time executes
the unload hook, but in some cases this runs when the loader's global
context has already been updated, resulting in the updated context being
freed and potential use-after-free bugs when the boot hook is subsequently
called.

This adds a new API, grub_loader_set_ex(), which allows a loader to specify
context that is passed to its boot and unload hooks. This is an alternative
to requiring that loaders call grub_loader_unset() before mutating their
global context.

Signed-off-by: Chris Coulson <chris.coulson@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/commands/boot.c | 66 +++++++++++++++++++++++++++++++++++++++++------
 include/grub/loader.h     |  5 ++++
 2 files changed, 63 insertions(+), 8 deletions(-)

diff --git a/grub-core/commands/boot.c b/grub-core/commands/boot.c
index bbca81e94..61514788e 100644
--- a/grub-core/commands/boot.c
+++ b/grub-core/commands/boot.c
@@ -27,10 +27,20 @@
 
 GRUB_MOD_LICENSE ("GPLv3+");
 
-static grub_err_t (*grub_loader_boot_func) (void);
-static grub_err_t (*grub_loader_unload_func) (void);
+static grub_err_t (*grub_loader_boot_func) (void *context);
+static grub_err_t (*grub_loader_unload_func) (void *context);
+static void *grub_loader_context;
 static int grub_loader_flags;
 
+struct grub_simple_loader_hooks
+{
+  grub_err_t (*boot) (void);
+  grub_err_t (*unload) (void);
+};
+
+/* Don't heap allocate this to avoid making grub_loader_set() fallible. */
+static struct grub_simple_loader_hooks simple_loader_hooks;
+
 struct grub_preboot
 {
   grub_err_t (*preboot_func) (int);
@@ -44,6 +54,29 @@ static int grub_loader_loaded;
 static struct grub_preboot *preboots_head = 0,
   *preboots_tail = 0;
 
+static grub_err_t
+grub_simple_boot_hook (void *context)
+{
+  struct grub_simple_loader_hooks *hooks;
+
+  hooks = (struct grub_simple_loader_hooks *) context;
+  return hooks->boot ();
+}
+
+static grub_err_t
+grub_simple_unload_hook (void *context)
+{
+  struct grub_simple_loader_hooks *hooks;
+  grub_err_t ret;
+
+  hooks = (struct grub_simple_loader_hooks *) context;
+
+  ret = hooks->unload ();
+  grub_memset (hooks, 0, sizeof (*hooks));
+
+  return ret;
+}
+
 int
 grub_loader_is_loaded (void)
 {
@@ -110,28 +143,45 @@ grub_loader_unregister_preboot_hook (struct grub_preboot 
*hnd)
 }
 
 void
-grub_loader_set (grub_err_t (*boot) (void),
-                grub_err_t (*unload) (void),
-                int flags)
+grub_loader_set_ex (grub_err_t (*boot) (void *context),
+                   grub_err_t (*unload) (void *context),
+                   void *context,
+                   int flags)
 {
   if (grub_loader_loaded && grub_loader_unload_func)
-    grub_loader_unload_func ();
+    grub_loader_unload_func (grub_loader_context);
 
   grub_loader_boot_func = boot;
   grub_loader_unload_func = unload;
+  grub_loader_context = context;
   grub_loader_flags = flags;
 
   grub_loader_loaded = 1;
 }
 
 void
+grub_loader_set (grub_err_t (*boot) (void),
+                grub_err_t (*unload) (void),
+                int flags)
+{
+  grub_loader_set_ex (grub_simple_boot_hook,
+                     grub_simple_unload_hook,
+                     &simple_loader_hooks,
+                     flags);
+
+  simple_loader_hooks.boot = boot;
+  simple_loader_hooks.unload = unload;
+}
+
+void
 grub_loader_unset(void)
 {
   if (grub_loader_loaded && grub_loader_unload_func)
-    grub_loader_unload_func ();
+    grub_loader_unload_func (grub_loader_context);
 
   grub_loader_boot_func = 0;
   grub_loader_unload_func = 0;
+  grub_loader_context = 0;
 
   grub_loader_loaded = 0;
 }
@@ -158,7 +208,7 @@ grub_loader_boot (void)
          return err;
        }
     }
-  err = (grub_loader_boot_func) ();
+  err = (grub_loader_boot_func) (grub_loader_context);
 
   for (cur = preboots_tail; cur; cur = cur->prev)
     if (! err)
diff --git a/include/grub/loader.h b/include/grub/loader.h
index b20864282..97f231054 100644
--- a/include/grub/loader.h
+++ b/include/grub/loader.h
@@ -40,6 +40,11 @@ void EXPORT_FUNC (grub_loader_set) (grub_err_t (*boot) 
(void),
                                    grub_err_t (*unload) (void),
                                    int flags);
 
+void EXPORT_FUNC (grub_loader_set_ex) (grub_err_t (*boot) (void *context),
+                                      grub_err_t (*unload) (void *context),
+                                      void *context,
+                                      int flags);
+
 /* Unset current loader, if any.  */
 void EXPORT_FUNC (grub_loader_unset) (void);
 
-- 
2.11.0